2009 cengage-wadsworth chapter 3 carbohydrates

2009 Cengage-Wadsworth

Chapter 3

Carbohydrates


Structural Features

• Simple carbohydrates– Monosaccharides– Disaccharides

• Complex carbohydrates– Oligosaccharides– Polysaccharides


Simple Carbohydrates

• Monosaccharides– Steroisomerism

• Chiral carbon - have 4 different atoms or groups attached to them

• Stereoisomers - have 2 or more chiral carbon atoms with same 4 groups attached but are not mirror images of each other



– Ring structures - molecules cyclize & form another chiral carbon

– Haworth models– Pentoses– Reducing sugars



• Disaccharides– Maltose– Lactose– Sucrose


Complex Carbohydrates

• Oligosaccharides– Raffinose– Stachyoses– Verbascose

• Polysaccharides– Starch– Glycogen– Cellulose


Digestion

• Polysaccharides– Salivary -amylase - mouth– Pacreatic -amylase - small intestine– Resistant starches

• Digestion of disaccharides– Disaccharidases - active in microvilli

of enterocytes


Absorption, Transport, & Distribution

• Absorption of glucose & galactose– Into cell: active transport - SGLT1– Into blood: diffusion, GLUT2

• Absorption of fructose– Into cell: facilitated transport - GLUT5– Into blood: GLUT2– Limited in 60% of adults



• Monosaccharide transport & cellular uptake

• Glucose transporters– GLUT isoforms

• Integral proteins• Each has specific combining site• Undergoes a conformational change upon

binding the molecule• Can reverse this change when unbound



– Specificity of GLUTs• GLUT1 - basic supply of glucose to cells• GLUT2 - low infinity transporter; glucose

from enterocyte to blood• GLUT3 - high-affinity for brain & other

glucose-dependent tissues• GLUT4 - insulin sensitive, in muscle &

adipose tissues• GLUT5 - for fructose



• Insulin– Role in cellular glucose absorption

• Binds to membrane receptor• Stimulates GLUT4 to move to membrane

• Maintenance of blood glucose levels


Glycemic Response to Carbohydrates

• Glycemic index– Increase in blood glucose during 2-

hour period after consumption of a certain amount of CHO compared with equal CHO from reference food

• Glycemic load– GI x g of CHO in 1 serving of food


Integrated Metabolism in Tissues

• Glycogenesis– Conversion of glucose to glycogen

• Glycogenolysis– Breakdown of glycogen to glucose– Phosphorolysis process– Regulation of phosphorylase

• Covalent - glucagon, epinephrine• Allosteric - AMP



• Glycolysis - degradation of glucose to pyruvate– Hexokinase/glucokinase reaction– Glucose phosphate isomerase– Phosphofructokinase reaction– Aldolase reaction– Glyceraldehyde 3-phosphate &

dihydroxyacetone phosphate



– Oxidation of glyceraldehyde 3-phosphate to carboxylic acid, incorporation of inorganic phosphate into high-energy anhydride bond

– Substrate-level phosphorylation of ADP– Phosphoglyceromutase– Dehydration of 2-phosphoglycerate– Phophoenolpyruvate (PEP) donates

phosphate group to ADP– Lacate dehydrogenase reaction



– Fructose enters pathway– Galactose is phophorylated– Galactose 1-phosphate converted to

glucose 1-phosphate– Glucose 6-phosphate enters

hexosemonophophate shunt– Glucose 1-phosphate enteres

glycogenesis– Glucose can enter glycolysis



• Substrate-level phosphorylation• The tricarboxylic acid cycle

– TCA pathway• Formation of citrate from oxaloacetate & acetyl

CoA• Isomerization of citrate to isocitrate• Dehydrogenation catalyzed by isocitrate

dehydrogenase• Decarboxylation & dehydrogenation of -

ketoglutarate



• Hydrolysis of thioester bond of acetyl CoA drives phosphorylation of guanosine diphosphate (GDP)

• Succinate dehydrogenase reaction

• Fumerase incorporates H2O across double bond of fumarate to form malate

• Malate converted to oxaloacetate

– ATPs produced by complete glucose oxidation

• C6H12O6 + 6O2 6CO2 + 6H2O + energy

• Yields 12 ATPs + 2 mol acetyl CoA per 1 mol glucose = 24 ATPs



– Acetyl CoA oxidation and tricarboxylic acid cycle intermediates

– NADH in anaerobic & aerobic glycolysis: the shuttle systems• Glycerol 3-phosphate shuttle system• Malate-aspartate shuttle system



• Formation of ATP– Biological oxidation & the electron

transport chain• Electron transport chain = sequential

reduction-oxidation• Oxidative phosphorylation = oxidation of

a metabolite by O2 through electron transport + phosphorylation of ADP



– Anatomical site for oxidative phosphorylation

– Components of the oxidative phosphorylation chain• Complex I NADH-coenzyme Q

oxidoreductase• Complex II• Complex III coenzyme Q-cytochrome c

oxidoreductase• Complex IV



– Phosphorylation of ADP to form ATP– Translocation of H+ – ATP synthase



• The hexosemonophosphate shunt (pentose phosphate pathway)– Pentose phosphates– Reduced cosubstrate NADPH

• Gluconeogenesis– Synthesis of glucose from non-CHO– Reversal of glycolytic pathway– Lactate utilization– Efficient glycogenesis


Regulation of Metabolism

• 4 mechanisms:– Negative or positive modulation of

allosteric enzymes– Hormonal activation by covalent

modification/induction– Directional shifts in reactions– Translocation of enzymes within cells



• Allosteric enzyme modulation– AMP, ADP, & ATP as allosteric

modulators– AMP’s positive modulation

• Causes shift from inactive to active form of phosphorylase b

• Stimulates phosphofructokinase



• Regulatory effect of NADH:NAD+ ratio

• Hormonal regulation– Glycolytic enzymes– Bifunctional enzymes– Gluconeogenic enzymes

• Directional shifts in reversible reactions


Perspective 3

Hypoglycemia: Fact or Fall Guy?


Hypoglycemia

• Preprandial vs. postprandial serum glucose levels

• Types:– Fasting hypoglycemia

• Usually caused by insulin, sulfonylureas

– Fed (reactive) hypoglycemia• Impaired glucose tolerance, idiopathic

postprandial syndrome

2009 cengage-wadsworth chapter 3 carbohydrates

Documents

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foodintegrated metabolism

equal cho

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mouthpacreatic amylase

conformational change